1. Technical Field
The present invention relates generally to a method of purifying aqueous dye solutions, and more particularly to removing aldehyde and/or ketone impurities from aqueous dye solutions to form dyes with an increase in purity.
2. Discussion
Certain dyes produced as aqueous solutions may be used in foodstuffs and packaging for foodstuffs. For example, the dye Color Index Acid Blue 9, which chemically is N-ethyl-N-[4-[[4-[ethyl[(3-sulfophenyl)methyl]amino]phenyl](2-sulfophenyl) methylene]-2,5-cyclohexadien-1-ylidene]3-sulfobenzenemethanaminium hydroxide inner salt, disodium salt, has been approved by the United States Food and Drug Administration for use in food, drugs, and cosmetics. Acid Blue 9 is currently used as a dye in various applications, such as in cleaning products, ponds, golf courses, and foodstuffs. When used as an additive in foodstuffs, however, certain strict standards of purity must be met.
Other dyes that are used in paper products for packaging foodstuffs, such as cardboard egg and apple cartons, must also have increased purity levels. For example, in the formation of Color Index Basic Violet 1, referred to as Methyl Violet, Michler's ketone, 4,4'-bis(dimethylamino)benzophenone, is produced as a by-product. Michler's ketone is a suspected carcinogen and must be removed from the Basic Violet 1 solution before the dye can be used with food packaging.
Dyes that are not intended to be used as food coloring agents may contain large amounts of impurities that do not affect the use of the dyes. When dyes are to be used as food coloring agents, they must be further purified. Typically, the dyes for food coloring are separated from the impurities by crystallization of the dyes. The equipment required to conduct this crystallization procedure, however, can be costly. Acid Blue 9 is particularly expensive to produce in food grade quality because it is highly soluble in water, resulting in a low yield of dye product after crystallization. An alternative method of removing impurities to produce food grade dyes is to salt out a dye from solution by adding a salt to the solution. This method can also be expensive because of the cost of the particular salt needed and loss of product. Additionally, the salting out method does not sufficiently increase the purity of a dye solution.
Other methods currently used for purifying dyes to food grade purity include nanofiltration and ultrafiltration. These procedures also have many disadvantages, including the requirement of specific equipment to conduct the procedures and the loss of product during the filtration process.
There thus exists a need in the art for a method of purification of aqueous dye solutions that are used as coloring agents for food and food packaging.